A structurally characterised pair of dicobalt(III) peroxo/superoxo complexes with C-2-symmetrical tetrapodal pentadentate amine ligands, and some reactivity en route

The tetrapodal pentaamine ligand 2,6-bis(1',3'-diamino-2'-methylprop-2'yl)p yridine (pyN(4), 1) provides square-pyramidal coordinated cobalt(II) buildi ng blocks, which have been used in the synthesis of the singly-bridged dico balt(III) mu(2)-eta(1):eta(1)-peroxo and superoxo complexes [(1)Co-O-2-Co(1 )](4+/5+), isolated as the chloride, bromide, and mixed chloride/dithionate and bromide/dithionate salts. The peroxo complex is accessible by the clas sical route involving oxygenation of [(pyN(4))Co-II], but has also been obt ained from a cobalt(III) precursor in air, which implies that the pentaamin e 1 acts as a multi-electron reductant. Oxidation of the peroxo complex wit h chlorine generated in situ from an HCl/H2O2 mixture (H2O2 derived from pa rtial peroxo complex hydrolysis) generates the superoxo complex. Both catio ns have highly symmetrical solid state structures, locked in transoid Co-O- O-Co conformations by two pairs of intramolecular hydrogen bonds. Each invo lves two protons in the equatorial Co(NH2R)(4) plane of one half of the mol ecule and the bridge oxygen atom in the other half. A significant differenc e between the two structures is the orientation of the O-2 bridge, which is coplanar with the pyridine rings of the coordination caps in the peroxo co mplex and at right angles in the superoxo complex. The reactivities of the complexes in acidic and basic media have been explored, and the mononuclear bromo complex [(1)CoBr](2+) was isolated in one of the products. Dithionit e, intended as an external reducing agent in the reaction of Na-3[COIII(CO3 )(3)] with 1, instead yields the S-sulfito complex cation [(1)Co(SO3)](+), by disproportionation of S2O42- to give SO32- and S2-. All complexes have b een characterised by H-1, C-13 NMR, IR, Raman, UV/Vis, and EPR spectroscopy las applicable), elemental analysis and X-ray structure determination, and the cyclic voltammetry parameters of the peroxo/superoxo pair of complexes have been determined.